The new security feature of Office365 is blocking access to .xml files when you try to view the e-mail message in Outlook web App. In the meantime you can see the attachment and download it using your Outlook client.
You can change the settings for which type are allowed/blocked using powershell cmdlets:
Get-OwaMailboxPolicy | Set-OwaMailboxPolicy -BlockedFileTypes @{Remove = ".xml"}
Get-OwaMailboxPolicy | Set-OwaMailboxPolicy -AllowedFileTypes @{Add = ".xml"}
Get-OwaMailboxPolicy | Set-OwaMailboxPolicy -BlockedMimeTypes @{Remove = "text/xml", "application/xml”}
Get-OwaMailboxPolicy | Set-OwaMailboxPolicy –AllowedMimeTypes @{Add = "text/xml", "application/xml”}